Selective hydroprocessing and mercaptan removal

ABSTRACT

A process for the production of naphtha streams from cracked naphthas having sulfur levels which help meet future EPA gasoline sulfur standards (30 ppm range and below).

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation in part of U.S. patentapplication Ser. No. 09/551,007 filed Apr. 18, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to a process for the production ofnaphtha streams from cracked naphthas having sulfur levels which helpmeet future EPA gasoline sulfur standards (30 ppm range and below).

BACKGROUND OF THE INVENTION

[0003] Environmentally driven regulatory standards for motor gasoline(mogas) sulfur levels will result in the widespread production of 120ppm S mogas by the year 2004 and 30 ppm by 2006. In many cases, thesesulfur levels will be achieved by hydrotreating naphtha produced fromFluid Catalytic Cracking (cat naphtha), which is the largest contributorto sulfur in the mogas pool. As a result, techniques are required thatreduce the sulfur in cat naphthas without reducing beneficial propertiessuch as octane.

[0004] Conventional fixed bed hydrotreating can reduce the sulfur levelof cracked naphthas to very low levels; however, such hydrotreating alsoresults in severe octane loss due to extensive reduction of the olefincontent. Selective hydrotreating processes such as SCANfining, a processdeveloped by ExxonMobil Research and Engineering Company, have recentlybeen developed to avoid massive olefin saturation and octane loss.Unfortunately, in such processes, the liberated H₂S reacts with retainedolefins forming mercaptan sulfur by reversion. Such processes can beconducted at severities which produce product within sulfur regulations;however, significant octane loss also occurs.

[0005] Hence, what is needed in the art is a process that producessulfur levels within regulatory amounts and which minimizes loss ofproduct octane.

SUMMARY OF THE INVENTION

[0006] In accordance with the present invention, there is provided amethod for producing a gasoline blendstock having a decreased amount ofsulfur comprising the steps of:

[0007] (a) selectively hydroprocessing a petroleum feedstream comprisingcracked naphtha and sulfur-containing species to produce a first naphthaproduct containing mercaptan sulfur having more than 5 carbon atoms,olefins, non-mercapatan sulfur, hydrogen sulfide, and hydrogen gas;

[0008] (b) removing at least a portion of said hydrogen sulfide and atleast a portion of said hydrogen gas from said first naphtha product toobtain a second naphtha product having a decreased amount of hydrogensulfide and hydrogen gas;

[0009] (c) contacting said second naphtha product with a liquidextractant effective for removing or converting at least a portion ofmercaptan sulfur in said second naphtha product to obtain a thirdnaphtha product having a decreased amount of said mercaptan sulfur; and

[0010] (d) fractionating said third naphtha product to obtain at leastone higher boiling point product comprising at least a portion of saidconverted mercaptan sulfur and at least one lighter boiling pointproduct.

[0011] In a preferred embodiment, step (b) above comprises separatinghydrogen gas and hydrogen sulfide from said first naphtha product byusing a separation device, such as a separation drum, to separatehydrogen and hydrogen sulfide from said first naphtha product, thenceconducting the first naphtha product, which comprises the treatednaphtha from the separation device to a monoethanolamine (MEA) scrubberto remove additional amounts of hydrogen sulfide, and conducting theremoved hydrogen gas and hydrogen sulfide from the separation device tofurther processing. The MEA and non-mercaptan sulfur are conducted toother plant process for further treatment and regeneration of the MEA.

[0012] In a preferred embodiment of the present invention, step (b)above comprises separating at least a portion of the hydrogen gas andhydrogen sulfide from the first naphtha product by using a separationdevice, such as a separation drum, to produce a second naphtha productdepleted in hydrogen sulfide and hydrogen gas. This second naphthaproduct is conducted to a monoethanolamine (MEA) scrubber to removeadditional amounts of hydrogen sulfide. The removed hydrogen gas andhydrogen sulfide from the separation device can be further processed.The MEA and non-mercaptan sulfur can be conducted to other plant processfor further treatment and regeneration of the MEA. The naphtha productfrom the MEA scrubber, the third naphtha product, is conducted to asweetening device to dimerize mercaptan to higher boiling disulfideswhich can then be removed by fractionation.

BRIEF DESCRIPTION OF THE FIGURES

[0013]FIG. 1 depicts the mercaptan reversion limits HDS of HCN using anRT-225 catalyst. The Y axis is product sulfur (wppm), product netproduct from mercaptans (wppm). The X axis is percent olefin saturation.

[0014]FIG. 2 depicts the mercaptan reversion limits HDS of HCN using aKF-742 catalyst. The Y axis is product sulfur (wppm), net product sulfurfrom mercaptans (wppm). The X axis is percent olefin saturation.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Hydrodesulfurization (HDS) processes are well known in the art.During such processes, an additional reaction occurs whereby thehydrogen sulfide produced during the process reacts with feed olefins toform alkylmercaptans. This reaction is commonly referred to as mercaptanreversion. Thus, to prevent such mercaptan reversion requires saturationof feed olefins resulting in a loss of octane.

[0016] It has been discovered, that the amount of mercaptan sulfur inthe reactor is controlled by the equilibrium established by the reactorexit temperature, exit olefin and 1-12S partial pressure, and that theSCANfining process can be run to produce an amount of mercaptan sulfurin the reactor that is often higher than the desired specificationamount while removing non-mercaptan sulfur to an acceptable regulatorylevel. Thus, by running the SCANfiner, or other selectivehydrodesulfurization process in such a manner, and combining it with asecond step to remove the undesirable mercaptans produced, regulatorysulfur levels can be met while retaining octane in the product produced.By selective hydrodesulfurization, it is meant a hydrodesulfurizationprocess runs in such a way as to remove sulfur while retaining a highlevel of olefins, thereby preventing unacceptable octane loss. As usedherein, “non-mercaptan sulfur” is meant to refer to organically boundsulfur species such as thiophenes, benzothiophenes, disulfides, etc.,that are not a result of mercaptan reversion.

[0017] Hence, in the instant invention, the product of the HDS unit,which will have a mercaptan sulfur content well above the desiredspecification but an acceptable non-mercaptan sulfur level(pre-determined), will be sent to a mercaptan removal step where atleast a portion of the mercaptans will be selectively removed, thereby,producing a product that meets specification.

[0018] Because the removal and/or conversion of at least a portion ofthe mercaptans is readily accomplished by the instant invention, it ispossible to operate the HDS unit to achieve a higher total sulfur levelwhile preserving a substantial amount of the feed olefins and octane.

[0019] For example, intermediate cat naphtha can be hydroprocessed to 60wppm total sulfur where approximately 45 wppm sulfur is mercaptansulfur. This first product would not meet the future 30 wppm sulfurspecification. This product would then be sent to a mercaptan removalstep where the sulfur level would be reduced to approximately 20 wppmtotal sulfur, meeting the specification. By not hydroprocessing thesample directly to 20 wppm sulfur, olefin saturation will be less thanis obtained from hydroprocessing to 20 wppm directly. Thus, considerableoctane is preserved affording an economical and regulatory acceptableproduct.

[0020] In the reactor, cat naphtha and hydrogen are passed over ahydroprocessing catalyst where organic sulfur is converted to hydrogensulfide and olefins are saturated to their corresponding paraffins. In atypical intermediate cat, naphtha >95% of the organic sulfur is inthiophene type structures. When HDS is conducted at conditions describedabove to retain olefins, hydrogen sulfide from thiophene HDS reacts withfeed olefins to form mercaptans. This mercaptan reversion was originallypostulated to predominantly occur in the reactor effluent train ratherthan in the reactor due to more favorable thermodynanucs. Hence, reactoreffluent train product residence times were controlled to controlmercaptan formation. The equilibrium constant at cold separatortemperature (100° F., 38° C.) is approximately 500 to 1600, whereas theequilibrium constant at reactor temperature (575° F., 302° C.) is 0.006to 0.03. Applicants discovered, upon a more rigorous examination of thethermodynamics of the system, that the level of product mercaptansobserved in pilot plants are thermodynamically allowed at reactortemperatures. Typical reactor ICN olefin partial pressures of 22 psi(152 kPa) would result in approximately 60 to 140 wppm sulfur asmercaptans, a result well above the currently proposed target of 30. Itwas clear from these thermodynamic calculations that mercaptan reversionis a limiting reaction for high selectivity cat naphtha hydroprocessingeven at the high temperature reactor conditions.

[0021] The extent and location for mercaptan reversion will. dependentirely on the relative reaction kinetics for the non-catalyzedreaction in the product recovery train vs. the catalyzed reaction thatwould occur in the reactor. It has been found that the rate of reactionunder reactor conditions is extremely rapid, producing thermodynamiclevels of mercaptans at very high space velocities, whereas thenoncatalyzed reaction is relatively slow even at higher than theexpected product recovery temperatures and H2S concentrations.

[0022] The HDS conditions needed to produce a hydrotreated naphthastream which contains non-mercaptan sulfur at a level below the mogasspecification as well as significant amounts of mercaptan sulfur willvary as a function of the concentration of sulfur and types of organicsulfur in the cracked naphtha feed to the HDS unit. Generally, theprocessing conditions will fall within the following ranges: 475-600° F.(246-316° C.), 150-500 psig (1136-3548 kPa) total pressure, 100-300 psig(7912170 kPa) hydrogen partial pressure, 1000-2500 SCFB hydrogen treatgas, and 110 LHSV.

[0023] The preferred hydroprocessing step to be utilized is SCANfining.However, other selective cat naphtha hydrodesulfurization processes suchas those taught by Mitsubishi (See U.S. Pat. Nos. 5,853,570 and5,906,730 herein incorporated by reference) can likewise be utilizedherein. SCANfining is described in National Petroleum RefinersAssociation paper # AM-99-31 titled “Selective Cat Naphtha Hydrofiningwith Minimal Octane Loss” and U.S. Pat. Nos. 5,985,136 and 6,013,598herein incorporated by reference. Selective cat naphtha HDS is alsodescribed in U.S. Pat. Nos. 4,243,519 and 4,131,537.

[0024] Typical SCANfining conditions include one and two stage processesfor hydrodesulfurizing a naphtha feedstock comprising reacting saidfeedstock in a first reaction stage under hydrodesulfurizationconditions in contact with a catalyst comprised of about 1 to 10 wt. %Mo0₃; and about 0.1 to 5 wt. % CoO; and a Co/Mo atomic ratio of about0.1 to 1.0; and a median pore diameter of about 60 A [Angstrom] to 200 Å[Angstrom]; and a Mo0₃ surface concentration in g Mo0₃/m² of about0.5×10⁻⁴ to 3×10⁻⁴; and an average particle size diameter of less thanabout 2.0 mm; and, optionally, passing the reaction product of the firststage to a second stage, also operated under hydrodesulfurizationconditions, and in contact with a catalyst comprised of at least oneGroup VIII metal selected from the group consisting of Co and Ni, and atleast one Group VI metal selected from the group consisting of Mo and W,more preferably Mo, on an inorganic oxide support material such asalumina.

[0025] In one possible flow plan for the invention, the SCANfiningreactor is run at sufficient conditions such that the difference betweenthe total organic sulfur (determined by x-ray adsorption) and themercaptan sulfur (determined by potentiometric test ASTM 3227) of theliquid product from the strippers is at or below the desired (target)specification (typically 30 ppm for non-mercaptan sulfur). This streamis then sent to a second step for removal of mercaptans.

[0026] In the mercaptan removal step, any technology known to theskilled artisan capable of removing >C5+mercaptan sulfur can beemployed. For example, sweetening followed by fractionation, thermaldecomposition, extraction, adsorption and membrane separation. Othertechniques which selectively remove C5+ mercaptan sulfur of the typeproduced in the first step may likewise be utilized.

[0027] One possible method of removing or converting the mercaptansulfur in accordance with step (c) of the instant process can beaccomplished by sweetening followed by fractionation. Such processes arecommonly known in the art and are described, for example, in U.S. Pat.No. 5,961,819. Processes relating to the treatment of sour distillatehydrocarbons are described in many patents. For example, U.S. Pat. Nos.3,758,404; 3,977,829 and 3,992,156 describe mass transfer apparatus andprocesses involving the use of fiber bundles which are particularlysuitable for such processes.

[0028] Other methods for accomplishing the mercaptan oxidation(sweetening) followed by fractionation are known and well established inthe petroleum refining industry. Among the mercaptan oxidation processeswhich may be used are the copper chloride oxidation process,Mercapfining, chelate sweetening and Merox, of which the Merox processis preferred because it may be readily integrated with a mercaptanextraction in the final processing step for the back end.

[0029] In the Merox oxidation process, mercaptans are extracted from thefeed and then oxidized by air in the caustic phase in the presence ofthe Merox catalyst, an iron group chelate (cobalt phthalocyanine) toform disulfides which are then redissolved in the hydrocarbon phase,leaving the process as disulfides in the hydrocarbon product. In thecopper chloride sweetening process, mercaptans are removed by oxidationwith cupric chloride which is regenerated with air which is introducedwith the feed to oxidation step.

[0030] The mercaptan oxidation process chosen by the practitioner of thepresent invention is not critical, but the one chosen must convert atleast a portion of the mercaptans to higher boiling disulfides which aretransferred to the higher boiling fraction, boiling above about 480° F.The higher boiling disulfides contained in the higher boiling fractionare then subjected to hydrogenative removal together with the thiopheneand other forms of sulfur present in the higher boiling portion of thecracked feed. This fractionation also results in at least one lighterboiling point product, in relation to the heavy boiling point product,boiling below about 480° F.

[0031] Mercaptan oxidation processes are described in Modern PetroleumTechnology, G. D. Hobson (Ed.), Applied Science Publishers Ltd., 1973,ISBN 085334 487 6, as well as in Petroleum Processing Handbook, Blandand Davidson (Ed.), McGraw-Hill, New York 1967, pages 3-125 to 3-130.The Merox process is described in Oil and Gas Journal 63, No. 1, pp.90-93 (January 1965). Reference is made to these works for a descriptionof these processes which may be used for converting the lower boilingsulfur components of the front end to higher boiling materials in theback end of the cracked feed.

[0032] Another method of removing the mercaptan sulfur in accordancewith step (c) will employ a caustic mercaptan extraction step. In theinstant invention, a combination of aqueous base and a phase transfercatalyst (PTC) known in the art will be utilized as the extractant or asufficiently basic PTC.

[0033] The addition of a phase-transfer catalyst allows for theextraction of higher molecular weight mercaptans (>C5+) produced duringhydrodesulfurization (HDS) into the aqueous caustic at a rapid rate. Theaqueous phase can then be separated from the petroleum stream by knowntechniques. Likewise, at least a portion of lower molecular weightmercaptans, if present, are also removed during the process.

[0034] Suitable phase transfer catalysts for use in the presentinvention can be either supported or unsupported. The attachment of thePTC to a solid substrate facilitates its separation and recovery andreduces the likelihood of contamination of the product petroleum streamwith PTC. Typical materials used to support PTC are polymers, silicas,aluminas and carbonaceous supports.

[0035] The PTC and aqueous base extractant may be supported on orcontained within the pores of a solid state material to accomplish themercaptan extraction. After saturation of the supported PTC bed withmercaptide in the substantial absence of oxygen, the bed can beregenerated by flushing with air and a stripper solvent to wash away thedisulfide which would be generated. If necessary, the bed could bere-activated with fresh base/PTC before being brought back on stream.This swing bed type of operation may be advantageous relative toliquid-liquid extractions in that the liquid-liquid separation stepswould be replaced with solid/liquid separations typical of solidadsorbent bed technologies. Note, the substantial absence of oxygen isrequired if seeking to remove mercaptans as opposed to sweetening theHDS product to disulfides. By substantial absence is meant no more thanthat amount of oxygen which will be present in a refinery processdespite precautions to exclude the presence of oxygen. Typically, 10 ppmor less, preferably 2 ppm or less oxygen will be the maximum amountpresent. Preferably, the process will be run in the absence of oxygen.

[0036] Such extractions include liquid-liquid extraction where aqueousbase and water soluble PTC are utilized to accomplish the extraction, orbasic aqueous PTC is utilized. A liquid-liquid extraction with aqueousbase and supported PTC where the PTC is present on the surface or withinthe pores of the support, for example a polymeric support; andliquid-solid extraction where both the basic aqueous PTC or aqueous baseand PTC are held within the pores of the support.

[0037] Thus, an “extractive” process whereby the thiols are firstextracted from the petroleum feedstream in the substantial absence ofair into an aqueous phase and the mercaptan-free petroleum feedstream isthen separated from the aqueous phase and passed along for furtherrefinery processing can be conducted. The aqueous phase may then besubjected to aerial oxidation to form disulfides from the extractedmercaptans. Separation and disposal of the disulfide would allow forrecycle of the aqueous extractant. Regeneration of the spent caustic canoccur using either steam stripping as described in The Oil and GasJournal, Sep. 9, 1948, pp. 95-103 or oxidation followed by extractioninto a hydrocarbon stream. Such extractants are easily selected by theskilled artisan and can include for example a reformate stream.

[0038] If it is desired to conduct a sweetening process, the extractionstep can be conducted in air; the loss of thiol is concurrent withgeneration of disulfide. This indicates a “sweetening process,” in thatthe total sulfur remains essentially constant in the feedstream, but themercaptan sulfur is converted to disulfide. Furthermore, the thiol istransported from the organic phase into the aqueous phase, prior toconversion to disulfide then back into the petroleum phase. We havefound this oxidation of mercaptide to disulfide to occur readily at roomtemperature without the addition of any other oxidation catalyst. Whenconducting a sweetening process, the extracting medium will consistessentially of aqueous base and PTC or aqueous basic PTC.

[0039] When utilizing a supported PTC, the porous supports may beselected from, molecular sieves, polymeric beads, carbonaceous solidsand inorganic oxides for example.

[0040] While not wishing to be limited by theory, the applicants hereofbelieve that, higher molecular weight mercaptans are extracted from thepetroleum feedstream into the basic solution which is contained withinthe pores of an appropriate solid support such as a “molecular sieve”.This is achieved by bringing into contact the solid-supported aqueousbasic solution with the petroleum stream by conventional methods such asare used in solid adsorbent technologies well known in the art. Uponcontact, the mercaptide anion should be generated and transported intothe aqueous phase within the pores of the molecular sieves. Themercaptan-free petroleum effluent stream is now ready for normalprocessing. With time, the capacity of the bed will be exceeded and thethiol content of the effluent will rise. At this point the bed will needto be regenerated. A second adsorbent bed will be swung into operation.Regeneration of the first bed will be accomplished by introduction ofoxygen (air) into the bed along with an organic phase which will providea suitable extractant stream for the disulfide which should form uponoxidation of the mercaptide anions. Such extractants are easily chosenby the skilled artisan. Pressure and heat could be used to stimulate theoxidative process. If necessary, the stripped bed could be regeneratedby re-saturation with fresh base/PTC solution before being swung backinto operation. Neither the base nor the PTC are consumed in thisprocess, other than by losses due to contaminants. The advantage ofusing a supported PTC is that the mercaptans are trapped within thepores of the support facilitating separation.

[0041] Bases utilizable in the extraction step are strong bases, suchas, for example, sodium, potassium and ammonium hydroxide, and sodiumand potassium carbonate, and mixtures thereof. These may be used as anaqueous solution of sufficient strength, typically base will be up to orequal to 50 wt. % of the aqueous medium, preferably about 15% to about25 wt. % when used in conjunction with onium salt PTCs and about 30-50wt. % when used in conjunction with polyethyleneglycol type PTCs.

[0042] The phase transfer catalyst is present in a sufficientconcentration to result in a treated feed having a decreased mercaptancontent. Thus, a catalytically effective amount of the phase transfercatalyst will be utilized. The phase transfer catalyst may be miscibleor immiscible with the petroleum stream to be treated. Typically, thisis influenced by the length of the hydrocarbyl chains in the molecule;and these may be selected by one skilled in the art. While this may varywith the catalyst selected, typically concentrations of about 0.01 toabout 10 wt. %, preferably about 0.05 to about 1 wt. % based on theamount of aqueous solution will be used.

[0043] Phase transfer catalysts (PTCs) suitable for use in this processinclude the types of PTCs described in standard references on PTC, suchas Phase Transfer Catalysis: Fundamentals, Applications and IndustrialPerspectives by Charles M. Starks, Charles L. Liotta and Marc Halpern(ISBN 0-412-04071-9 Chapman and Hall, 1994). These reagents aretypically used to transport a reactive anion from an aqueous phase intoan organic phase in which it would otherwise be insoluble. This“phase-transferred” anion then undergoes reaction in the organic phaseand the phase transfer catalyst then returns to the aqueous phase torepeat the cycle, and hence is a “catalytic” agent. In the invention, itis believed that, the PTC transports the hydroxide anion, —OH, into thepetroleum stream, where it reacts with the thiols in a simple acid basereaction, producing the deprotonated thiol or thiolate anion. Thischarged species is much more soluble in the aqueous phase and hence theconcentration of thiol in the petroleum stream is reduced by thischemistry.

[0044] A wide variety of PTC would be suitable for this application.These include onium salts such as quaternary ammonium and quaternaryphosphonium halides, hydroxides and hydrogen sulfates for example. Whenthe phase transfer catalyst is a quaternary ammonium hydroxide, thequaternary ammonium cation will preferably have the formula:

[0045] where q=1/w+1/x+1/y+1/z and wherein q>1.0. Preferably, q>3. Inthis formula, Cw, Cx, Cy, and Cz represent alkyl radicals with carbonchain lengths of w, x, y and z carbon atoms, respectively. The preferredquaternary ammonium salts are the quaternary ammonium halides.

[0046] The four alkyl groups on the quaternary cation are typicallyalkyl groups with total carbons ranging from four to forty, but may alsoinclude cycloalkyl, aryl, and arylalkyl groups. Some examples of useableonium cations are tetrabutyl ammonium, tetrabutylphosphonium,tributylmethyl ammonium, cetyltrimethyl ammonium, methyltrioctylammonium, and methyltricapryl ammonium. In addition to onium salts,other PTC have been found effective for hydroxide transfer. Theseinclude crown ethers such as 18-crown-6 and dicyclohexano-18-crown-6 andopen chain polyethers such as polyethyleneglycol 400. Partially-cappedand fully-capped polyethyleneglycols are also suitable. This list is notmeant to be exhaustive but is presented for illustrative purposes.Supported or unsupported PTC and mixtures thereof are utilizable herein.

[0047] The amount of aqueous medium to be added to the petroleum streambeing treated will range from about 5% to about 200% by volume relativeto petroleum feed.

[0048] While process temperatures for the extraction of from 25° C. to180° C. are suitable, lower temperatures of less than 25° C. can be useddepending on the nature of the feed and phase transfer catalyst used.The pressure should be sufficient pressure to maintain the petroleumstream in the liquid state. Oxygen must be excluded, or be substantiallyabsent, during the extraction and phase separation steps to avoid thepremature formation of disulfides, which would then redissolve in thefeed. Oxygen is necessary for a sweetening process.

[0049] Following the extraction of the mercaptans, and separation of themercaptan free petroleum stream, the stream is then passed through theremaining refinery processes, if any. The base and PTC or basic PTC maythen be recycled for extracting additional mercaptans from a freshhydrodesulfurized petroleum stream.

[0050] The mixture of PTC and base may consist essentially of or consistof PTC and base. When using basic PTCs, they may consist essentially ofor consist of basic PTCs. Preferably, the invention will be practiced inthe absence of any catalyst other than the phase transfer catalyst suchas those used to oxidize mercaptans, e.g., metal chelates as describedin U.S. Pat. Nos. 4,124,493; 4,156,641; 4,206,079; 4,290,913; and4,337,147. Hence in such cases the PTC will be the only catalystpresent.

[0051] The conditions under which the HDS unit is operated are chosensuch that at least a portion of the organic sulfur species present inthe feed (e.g., thiophenes, benzothiophenes, mercaptans, sulfides,disulfides and tetrahydrothiophenes) are substantially converted intohydrogen sulfide without significantly impacting olefin saturation. Bythis it is meant, that the conditions chosen are sufficient toaccomplish the conversion of organic sulfur in the feed. Olefinsaturation will thus, only occur to the extent caused by the HDS organicsulfur conversion conditions. These conditions are easily selected bythe skilled artisan.

[0052] Once the naphtha has at least a portion of the organo sulfur andmercaptans removed therefrom is separated from the extractant mixture,the extractant mixture can then be recycled to extract a freshhydroprocessed stream. The preferred streams treated in accordanceherewith are naphtha streams, more preferably, intermediate naphthastreams. Regeneration of the spent caustic can occur using either steamstripping as described in The Oil and Gas Journal, Sep. 9, 1948,pp.-95-103 or oxidation followed by extraction into a hydrocarbonstream.

[0053] Typically regeneration of the inercaptan containing causticstream is accomplished by mixing the stream with an air stream suppliedat a rate which supplies at least the stoichiometric amount of oxygennecessary to oxidize the mercaptans in the caustic stream. The air orother oxidizing agent is well admixed with the liquid caustic stream andthe mixed-phase admixture is then passed into the oxidation zone. Theoxidation of the mercaptans is promoted through the presence of acatalytically effective amount of an oxidation catalyst capable offunctioning at the conditions found in the oxidizing zone. Severalsuitable materials are known in the art.

[0054] Preferred catalysts include a metal phthalocyanine such as cobaltphthalocyanine or vanadium phthalocyanine, etc. Higher catalyticactivity may be obtained through the use of a polar derivative of themetal phthalocyanine, especially the monosulfo, disulfo, trisulfo, andtetrasulfo derivatives.

[0055] The preferred oxidation catalysts may be utilized in a form whichis soluble or suspended in the alkaline solution or it may be placed ona solid carrier material. If the catalyst is present in the solution, itis preferably cobalt or vanadium phthalocyanine disulfonate at aconcentration of from about 5 to 1000 wt. ppm. Carrier materials shouldbe highly absorptive and capable of withstanding the alkalineenvironment. Activated charcoals have been found very suitable for thispurpose, and either animal or vegetable charcoals may be used. Thecarrier material is to be suspended in a fixed bed which providesefficient circulation of the caustic solution. Preferably the metalphthalocyanine compound comprises about 0.1 to 2.0 wt. % of the finalcomposite.

[0056] The oxidation conditions utilized include a pressure of fromatmospheric to about 6895 kPag (1000 psig). This pressure is normallyless than 500 kPag (72.5 psig). The temperature may range from ambientto about 95 degrees Celsius (203 degrees Fahrenheit) when operating nearatmospheric pressure and to about 205 degrees Celsius (401 degreesFahrenheit) when operating at superatmospheric pressures. In general, itis preferred that a temperature within the range of about 38 to about 80degrees Celsius is utilized.

[0057] To separate the mercaptans from the caustic, the pressure in thephase separation zone may range from atmospheric to about 2068 kPag (300psig) or more, but a pressure in the range of from about 65 to 300 kpagis preferred. The temperature in this zone is confined within the rangeof from about 10 to about 120 degrees Celsius (50 to 248 degreesFahrenheit), and preferably from about 26 to 54 degrees Celsius. Thephase separation zone is sized to allow the denser caustic mixture toseparate by gravity from the disulfide compounds. This may be aided by acoalescing means located in the zone.

[0058] Another possible means for conducting step (c) of the processinvolves catalytic decomposition. The catalytic decomposition ofmercaptans to form olefins and H2S at high temperature vapor conditionsis well known in the art. Simple, noncatalyzed thermal decomposition iswell known to be quite slow for primary mercaptans (W. M. Malisoff andE. M. Marks, Industrial and Engineering Chemistry 1931, 23, pp.1114-1120), requiring temperatures in excess of 400° C. in order toachieve greater than 10% conversion. A catalyst is therefore preferred.A wide variety of solid oxides are well known to catalyze this reaction.Typical materials utilized to catalyze this reaction are described in C.P. C. Bradshaw and L. Turner British Patent No. 1,174,407, December1969. For example 32% conversion of 2-butanethiol is obtained over analumina catalyst at 250° C.; LHSV of 6 and 1 atmosphere. Mixed solidoxides, such as amorphous and crystalline silica-alumina are also wellknown to catalyze this reaction. Although traditional metal sulfidecatalyst are also suitable for this reaction, a solid oxide would bepreferred due to the absence of a olefin hydrogenation function on thecatalyst.

[0059] For example, the catalyst may be selected from: alumina, silica,titania, Group IIA metal oxides, mixed oxides of aluminum and Group IIAmetals, silica—alumina, crystalline silica-alumina, aluminum phosphates,crystalline aluminum phosphates, silica-alumina phosphates, Group VImetal sulfides, and Group VIII metal promoted Group VI metal sulfidesand mixtures thereof.

[0060] The preferred catalyst may be selected from: alumina, silica,titania, Group IIA metal oxides, mixed oxides of aluminum and Group IIAmetals, silica-alumina, crystalline silica-alumina, aluminum phosphates,crystalline aluminum phosphates, silica-alumina phosphates and mixturesthereof. The most preferred catalyst is alumina.

[0061] In one embodiment of this invention, the reactor effluent fromSCANfining is condensed in a separation drum, and at least a portion ofthe gaseous products of the HDS reaction such as, for example, H₂S areseparated from the liquid product. The liquid product is then sent to astripper or stabilizer vessel where at least a portion of the dissolvedH₂S and light hydrocarbons are removed. The liquid from thestripper/stabilizer is then heated to vaporization at a pressure betweenatmospheric. pressure and 200 psig (1480 kPa). This vapor feed andhydrogen are then sent to an additional mercaptan decomposition reactoroperated at effective conditions that contains a catalyst suitable fordecomposing the mercaptans. By effective conditions it is meantconditions under which at least a portion of the mercaptans isdecomposed and the saturation of feed olefins is kept to a minimum.Non-limiting examples of suitable catalysts are described above. Typicaltemperatures for this reactor would be temperatures of about 200-450°C., pressures from atmospheric to about 200 psig and hydrogen treatrates of about 100-5000 SCFB. It is understood that the temperature andpressure chosen must be such as to produce a complete vaporous feed tothe reactor. Subsequent to the reaction the product containing reducedlevels of mercaptans is condensed in another separation drum and thenstripped of any remaining dissolved H₂S in an additional stripper.

[0062] In a second embodiment of this invention the mercaptandecomposition reactor is placed immediately following the firstseparation drum and sent without stripping directly to the mercaptandecomposition reactor at the conditions described above. This embodimentremoves the requirement for an intermediate stripper. Although thisconfiguration will result in some H₂S in the mercaptan destructionreactor, this can be overcome by running the mercaptan reactor atslightly higher temperatures and/or lower pressures to compensate and isreadily accomplished by the skilled artisan.

[0063] In another embodiment of the present invention, the reactoreffluent from SCANfining is condensed in a separation drum, and at leasta portion of the gaseous products of the HDS reaction such as, forexample, H₂S are separated from the liquid product. The liquid productis then sent to another sulfur removal step such as a stripper, scrubberor stabilizer vessel, preferably an MEA scrubber, to remove anadditional portion of hydrogen sulfide. After removal of at least anadditional portion of the hydrogen sulfide, the effluent with reducedamounts of hydrogen sulfide is sent to a reactor to remove or convert atleast a portion of the mercaptan sulfur. The effluent from the mercaptanremoval/conversion reactor is then sent to the existing stripper of thebase hydrotreating unit where at least a portion of the convertedmercaptans are removed as a heavier boiling point fraction. In thestripper, a countercurrent or co-current, in relation to the flow of thenaphtha product stream of the mercaptan conversion reactor, hydrocarbonstream may be added, preferably diesel oil, to promote the removal of atleast a portion of the converted mercaptans. The injection of ahydrocarbon stream may be necessary because of low concentrations ofconverted mercaptans.

[0064] Thus, the process may involve three steps. First, a crackednaphtha, which may be a cat naphtha, coker naphtha, steam crackednaphtha or a mixture thereof, containing quantities of undesirablesulfur species and desirable high octane olefinic species is treated ina selective hydrotreating process (for example SCANfining). Theselective hydrotreating process removes at least a portion of mercaptanand non-mercaptan (e.g., thiophenic) sulfur species from the feed with aminimum saturation of olefins. During this desulfurization process, H₂Sis liberated and reacts with olefins in the naphtha product to formmercaptans. Conditions in the selective naphtha hydrotreating processare chosen to reduce the level of non-mercaptan sulfur species in theproduct to preferably less than 30 wppm. The second step involves theremoval of at least a portion of hydrogen sulfide and light end, C₄C₁,hydrocarbons through the use of a separation drum and MEA scrubber. Thethird step involves removing at least a portion of the mercaptans formedin the first step. A variety of techniques can be used to accomplishthis while minimizing olefin saturation and hence octane lost. Theseinclude: sweetening and fractionation; extraction, adsorption, mildhydrotreating, and thermal decomposition. The final naphtha product fromthe three step sequence has very low sulfur content (i.e., 30 ppm orless) and increased octane.

[0065] The product from the instant process is suitable for blending tomake motor gasoline (mogas) that meets sulfur specifications in the 30ppm range and below.

[0066] The following examples, which are meant to be illustrative andnot limiting, illustrate the potential benefit of the invention, byshowing specific cases in which a selective hydrofining process has beenoperated to produce varying levels of total and mercaptan sulfur. Byreference to these cases, it should be apparent that coupling suchselective hydrotreating with a subsequent mercaptan removal technologywill result in improved ability to produce low sulfur products withreduced losses of olefins and octane.

EXAMPLE 1

[0067] A sample of naphtha product from a commercial Fluid CatalyticCracking unit was fractionated to provide an intermediate cat naphtha(ICN) stream having a nominal boiling range of 180-370° F. The ICNstream contained 3340 wppm sulfur and 32.8 vol. % olefins (measured byFIA) and had a Bromine number of 50.7. The ICN stream was hydrotreatedat SCANfining conditions using RT-225 catalyst at 500° F., 250 psig,1500 SCFB hydrogen treat gas and 0.5 LHSV. The SCANfiner productcontained 93 wppm sulfur and had a Bromine number of 19.4. Of the 93wppm sulfur, 66 wppm was mercaptan sulfur and the remainder wasnon-mercaptan sulfur. The SCANfiner product was sweetened by contactingit in air with a solution of 20 wt. % NaOH in water and 500 wppmcetyltrimethylammonium bromide in water. The resulting sweetenedSCANfiner product contained 5 wppm mercaptan sulfur. The sweetenedSCANfiner product was then fractionated via a 15/5 distillation toachieve a 350° F. cut point. 90 wt. % was recovered as 350° F.desulfurized product which contained 21 wppm total sulfur, 5 wppmmercaptan sulfur and had a Bromine number of 19.5. The remaining 350° F.product contained 538 wppm sulfur consisting primarily of high boilingdisulfides from the sweetening step. The desulfurized 350° F. product issuitable for blending into low sulfur gasoline. The 350° F. product canbe processed further via hydrotreating to remove the disulfides.

COMPARATIVE EXAMPLE

[0068] The ICN stream of Example 1 was hydrotreated at SCANfiningconditions using RT-225 catalyst at 525° F., 227 psig, 2124 SCFBhydrogen treat gas and 1.29 LHSV. The SCANfiner product contained 35wppm sulfur and had a Bromine number of 10.1. Although this SCANfinerproduct had <50 ppm S total sulfur content like the 350° F.—product ofExample 1, the Bromine number was significantly lower (10.1 vs. 19.5)indicating the olefin content was lower resulting in increased octaneloss.

EXAMPLE 2

[0069] A commercially prepared, catalyst (RT-225) consisting of 4.34 wt.% Mo0₃, 1.19 wt. % CoO. SCANfining operation was demonstrated using acatalyst in a commercially available 1.3 mm asymmetric quadralobe sizewith a Heavy Cat Naphtha feed, 2125 wppm total sulfur, and 27.4 brominenumber, in an isothermal, downflow, all vapor-phase pilot plant.Catalyst volume loading was 35 cubic centimeters. Reactor conditionswere 560° F., 2600 scf/b, 100% hydrogen treat gas and 300 psig totalinlet pressure. Due to small random changes that occurred whileadjusting pump settings, space velocity was varied between 3 and 5 LHSV(defined as volume of feed per volume of catalyst per hour). Overallsulfur removal levels ranged between 93.9 and 98.5 wt. % and olefinsaturation between 21.9 and 35.8 wt. %. FIG. 1, shows product sulfurlevels, both total and product sulfur less mercaptan sulfur, as afunction of olefin saturation. To make 30 ppm sulfur in the productwithout mercaptan sulfur removal would require approximately 34 wt. %olefin hydrogenation compared to 26.5 wt. % with mercaptan removal. Iflower sulfur levels were required, this difference in olefinhydrogenation would be even higher. It should be noted that the threelowest sulfur data points at the highest olefin saturation or brominenumber removal were obtained near the start of the pilot plant run (11to 13 days on cat naphtha). It is known that as the catalyst ages orcokes, selectivity for sulfur removal over olefin hydrogenation isimproved. As a result, this example may slightly exaggerate thepotential benefit of mercaptan sulfur removal post SCANfining since theother data points were collected near end of run (29 to 33 days on catnaphtha).

EXAMPLE 3

[0070] A commercially prepared, reference batch of KF-742 (10 cc charge)conventional hydrotreating catalyst was used in this test. The catalyst(KF-742) consisted of 15.0 wt. % Mo0₃, 4.0 wt. % CoO. The SCANfiningoperation was demonstrated using a catalyst in a commercially available1.3 mm asymmetric quadralobe size with a Heavy Cat Naphtha feed, 2125wppm total sulfur, and 27.4 bromine number in an isothermal, downflow,all vapor-phase pilot plant. Reactor conditions were 560° F., 2600scf/b, 100% hydrogen treat gas and 300 psig total inlet pressure. Forthis test, space velocity was adjusted between 7 and 28 LHSV and all ofthe data was collected near end of run (30 to 38 days on cat naphtha).Each day, a small decrease in feed rate was made. Overall sulfur removallevels ranged between 92.5 and 99.2% and olefin saturation between 21.9and 35.8%. FIG. 2, shows product sulfur levels, both total and productsulfur less mercaptan sulfur, as a function of olefin saturation. Tomake 30 ppm sulfur in the product without mercaptan sulfur removal wouldrequire approximately 40% olefin hydrogenation compared to 33%. If lowersulfur levels were required, this difference in olefin hydrogenation oroctane loss would be even higher. It should be noted that for the lasttwo points, measured mercaptan sulfur was slightly greater than totalsulfur measured. As a result, all sulfur was assumed to be mercaptan.

EXAMPLE 4

[0071] A sample of ICN (3340 wppm total sulfur and 50.7 bromine number)was SCANfined in an isothermal, downflow, all vapor-phase pilot plantusing RT-225 high dispersion catalyst mentioned in Example 1. Examplesare shown in Table I below which shows that mercaptan reversion productsform a large percentage of the remaining product sulfur. TABLE 1Examples of Mercaptan Reversion Balance 9 12 23 Reactor Operation Temp °C. 274 302 274 Pressure kPa 1653 1653 1653 LHSV 1.15 3.5 2.5 Treat gasrate 2200 2200 2200 scf/bbl Product Analysis Total Sulfur 34 38 7Mercaptan sulfur 33.2 32.4 88.5

EXAMPLE 5

[0072] A previously hydroprocessed intermediate cat naphtha containing60 wppm total sulfur, 43 wppm sulfur as mercaptan and a bromine numberof 19.3 was subjected to catalytic mercaptan destruction over ag-alumina catalyst in fixed bed microreactor at the followingconditions. As can be seen by the data below extremely high mercaptanconversions (>90%) is achieved at almost all of the vapor conditionsshown. It is also obvious from the data that higher temperatures andtreat rates favor mercaptan decomposition. TABLE 2 CatalyticDecomposition of Mercaptans in Intermediate Cat Naphtha over g-AluminaTemp ° C. 250 300 300 300 300 300 300 Pressure 446 446 446 446 446 446446 (kPa) H₂ treat rate 5400 5400 1700 1700 1700 850 850 LHSV 1.0 1.01.0 2.0 4.0 4.0 4.0 Wt. % 98 100 95 97 95 91 84 Mercaptan Decomposed

What is claimed:
 1. A method for producing a gasoline blendstock havinga decreased amount of sulfur comprising the steps of: (a) selectivelyhydroprocessing a petroleum feedstream comprising cracked naphtha andsulfur-containing species to produce a first naphtha product comprisingmercaptan sulfur having more than 5 carbon atoms, olefins, non-mercaptansulfur, and hydrogen gas; (b) removing at least a portion of saidhydrogen sulfide and at least a portion of said hydrogen gas from saidfirst naphtha product to obtain a second naphtha product having adecreased amount of hydrogen sulfide and hydrogen gas; (c) contactingsaid second naphtha product with a liquid extractant and removing orconverting at least a portion of said mercaptan sulfur from said secondnaphtha product to obtain a third naphtha product having a decreasedamount of said mercaptan sulfur; and (d) fractionating said thirdnaphtha product to obtain at least one higher boiling point productcomprising at least a portion of said converted mercaptan sulfur and atleast one lighter boiling point product.
 2. The method of claim 1wherein said lighter boiling point product has a boiling point belowabout 480° F., and said higher boiling point product has a boiling pointabove about 480° F.
 3. The method of claim 1 wherein said first naphthaproduct contains less than 50 ppm non-mercaptan sulfur.
 4. The method ofclaim 1 wherein said first product contains less than 30 wppmnon-mercaptan sulfur.
 5. The method of claim 1 wherein said removal step(c) is accomplished by a process selected from the group consisting ofextraction, adsorption, fractionation, sweetening followed byfractionation, thermal decomposition and membrane separation.
 6. Themethod according to claim 5 wherein said mercaptan sulfur is removed orconverted by sweetening in the sweetening unit already existing with thebase hydrotreating unit.
 7. The method according to claim 1 whereinabout 1% to about 100% of said hydrogen gas is removed from said firstnaphtha product.
 8. The method according to claim 1 wherein step (d) isconducted in a disulfide fractionator.
 9. The method according to claim1 wherein step (d) is conducted in the existing stripper of the basehydrotreating unit.
 10. The method of claim 2 wherein step (b) of claim1 comprises: (a) separating from said first naphtha product at least aportion of hydrogen gas and at least a portion of hydrogen sulfide in aseparation drum to produce a naphtha product having reduced levels ofhydrogen sulfide and hydrogen gas; (b) passing said naphtha producthaving reduced levels of hydrogen sulfide and hydrogen gas to amonoethanolamine scrubber to produce a naphtha product having reducedlevels of non-mercaptan sulfur; and, (c) regenerating said monoethanolamine.
 11. The method of claim 8 or 9 wherein a hydrocarbon stream isinjected into said existing stripper such that the flow of saidhydrocarbon stream is countercurrent or co-current to the flow of theproduct stream from the mercaptan conversion reactor.
 12. The methodaccording to claim 11 wherein said hydrocarbon stream is diesel oil.